Conventionally, two-stage compression refrigeration systems, which have a refrigerant circuit in which a two-stage compressor, a condenser, a two-stage expansion device, and an evaporator are sequentially connected, have been used in the art. In order to achieve a high coefficient of performance (COP), such two-stage compression refrigeration systems use an economizer for separating a gas refrigerant from a gas-liquid two-phase refrigerant, and guiding the gas refrigerant to a medium pressure portion of the two-stage compressor (see, e.g., Patent Document 1).
An economizer shown in FIG. 2 of Patent Document 1 includes a tank having an introducing portion for introducing a refrigerant into the tank, a gas outlet for guiding a separated gas refrigerant to a two-stage compressor, and a liquid outlet for guiding a separated liquid refrigerant to an evaporator. Inside the tank is provided a float expansion valve that is attached to the liquid outlet to adjust the throttle amount according to the level of the liquid refrigerant in the tank.